The Mixed Lineage Leukemia (Mil) gene encodes a chromatin modifying protein that can be deregulated by many different chromosomal translocations to result in leukemia. We have shown that the murine Mil gene is essential for the development of hematopoietic stem cells (HSCs) during embryogenesis, and that a significant overlap exists between the target genes regulated by Mil and its corresponding fusion oncogenes. To understand the functions of Mil within the hematopoietic system, we propose to systematically test the effect of deleting this gene within defined populations of the hematopoietic system. Our preliminary data suggests a profound requirement for Mil in the maintenance of bone marrow hematopoiesis, but the molecular basis for this requirement or the precise cell types affected is not known. We propose to 1) utilize a conditional knockout model to determine whether Mil regulates HSCs in the bone marrow through effects on self-renewal, proliferation or homing/migration, 2) identify M/-dependent processes that maintain multipotent progenitors, and 3) whether re-expression of individual Hox target genes replace these functions Our long-term goals are to understand the genetic networks that operate to balance HSC self-renewal, proliferation/quiescence and differentiation. In particular we are interested in how the MII-Hox pathway integrates with other known transcriptional regulatory pathways to regulate hematopoiesis. Furthermore, it is our expectation that by dissecting the precise role of Mil in hematopoiesis, we will understand better how to target MLL fusion oncogenes that deregulate these normal processes. Relevance: Our work is focused on understanding the gene regulatory pathways that control the identity and function of hematopoetic stem cells and their differentiated progeny. The same regulatory pathways are perturbed in leukemia, so understanding the regulation of these pathways will be essential for devising new strategies to treat leukemia.